The VIR thermostatic expansion valve controls the
flow of refrigerant to the evaporator by sensing the
temperature and pressure of the refrigerant gas, as it
passes through the VIR unit on its way to the
compressor. The POA valve controls the flow of
refrigerant from the evaporator to maintain a constant
evaporator pressure of 30 psi. The VIR and the POA
valves are capsule type of valves. When found to be
defective, you must replace the complete valve
The drier desiccant is in a bag in the receiver shell.
It is replaceable by removing the shell and removing
the old bag and installing a new bag of desiccant.
Service procedures for the VIR system differ in
some respect from the service procedures performed
on conventional automotive air-conditioning systems.
Observe the following precautions whenever you
are tasked to service air-conditioning equipment:
A system that has been opened to replace a
component or one which has discharged through
leakage must be evacuated before charging.
Never open or loosen a connection before
discharging the system.
Immediately after disconnecting a component
from the system, seal the open fittings with a cap
Before disconnecting a component from the
system, clean the outside of the fittings
Do not remove the sealing caps from a
replacement component until you are ready to
Refrigerant oil absorbs moisture from the
atmosphere if it is left uncapped. Do not open an
oil container until it is ready to use, and install
the cap immediately after using. Store the oil
only in a clean, moisture-free container.
Before connecting to an open fitting, always
install a new seal ring. Coat the fitting and seal
with the refrigerant oil before connecting.
When installing a refrigerant line, avoid sharp
bends. Position the line away from the exhaust
or any sharp edges that may chafe the line.
Tighten the fittings only to specified torque. The
copper and aluminum fittings that are used in
refrigerant systems will not tolerate over-
When disconnecting a fitting, use a wrench on
both halves of the fitting to prevent twisting of
refrigerant lines or tubes.
Do not open a refrigerant system or uncap a
replacement component unless it is as close as
possible to room temperature. This prevents
condensation from forming inside a component
that is cooler than the surrounding air.
Keep the service tools and work area clean.
Contamination of a refrigerant system through
careless work habits must be avoided.
DIAGNOSIS, TESTING, AND SERVICING
Diagnosis is more than just following a series of
interrelated steps to find the solution to a specific
condition. It is a way of looking at systems that are not
functioning the way they should and finding out why.
Also, it is knowing how the system should work and
whether it is working correctly.
diagnosticians use the same basic procedures.
There are basic rules for diagnosis. If these rules
are followed, the cause of the condition will usually be
found the first time through the system.
1. Know the system; know how the parts go
together. Also, know how the system operates and its
limits, and what happens when something goes wrong.
Sometimes this means comparing a system that is
working properly with the one you are servicing.
2. Know the history of the system. How old or new
is the system? What kind oftreatment has it had? Has it
been serviced in the past in such a manner that might
relate to the present condition? What is the service
history? A clue in any of these areas might save a lot of
3. Know the probability of certain conditions
developing. It is true that most conditions are caused by
simple things, rather than by complex ones, and they
occur in a fairly predictable pattern. Electrical problem
conditions, for instance, usually occur at connections,
rather than in components. An engine "no-start" is
more likely to be caused by a loose wire or some
component out of adjustment than a sheared off
camshaft. Know the difference between impossible and
improbable. Many good technicians have spent hours
diagnosing a system because they thought certain
failures were "impossible," only to find out the failures